Process for the treatment of organic pollutants in wastewaters by wet oxidation

ABSTRACT

Organic pollutants in wastewaters are wet-oxidized by addition of pure oxygen or an oxygen-containing gas at temperatures of 80° to 330° C., under pressures of 1 bar to 200 bar and at a pH value below 7. For the wet oxidation process, iron ions and digested sludge or surplus sludge from a biological sewage treatment plant are added to the wastewater.

This invention relates to a process for the degradation of organicpollutants in wastewaters by wet oxidation in which pure oxygen or anoxygen-containing gas is added at temperatures of 80° to 330° C. andunder pressures of 1 bar to 200 bar.

To reduce the temperatures (and hence the pressures) required foroxidation, it has already been proposed to treat the organicallypolluted wastewaters with oxygen or oxygen-containing gases at a redoxpotential of 300 to 600 mV, the redox potential being adjusted byaddition of redox systems, preferably Fe⁺⁺ or Fe⁺⁺⁺ ions. It is possiblein this way to reduce the reaction temperature to well below 250° C. Itis also known that the reaction temperature can be further reduced byadding benzoquinones or naphthoquinones as co-catalysts to thewastewater during the wet oxidation process. This process is describedin DE 33 16 265 and in the Article by O. Horak in Chem. Ing. Techn. 62(1990), No. 7, pages 555-557.

It has now been found that treated sludge can be used as co-catalystinstead of the quinones mentioned in DE 33 16 265. The treated sludgemay be either surplus sludge from an industrial sewage treatment plantor digested sludge from a communal sewage treatment plant. There is thusno longer any need for the expensive quinoidal chemicals or for theelaborate pretreatment measures described in DE 33 16 265. At the sametime, a large part of the biological surplus sludge can be oxidativelyeliminated in this way.

Accordingly, the present invention relates to a process for thedegradation of organic pollutants in wastewaters by wet oxidation inwhich pure oxygen or an oxygen-containing gas is added at temperaturesof 80° to 330° C. and preferably 120° to 200° C., under a pressure of 1bar to 200 bar and preferably 3 bar to 50 bar and at a pH value below 7and preferably below 4, characterized in that the wet-oxidativedegradation is carried out in the presence of iron and digested sludgeor surplus sludge from a biological sewage treatment plant.

The digested sludge or the surplus sludge is preferably added to thewastewater in a quantity of 1 g to 150 g and more preferably in aquantity of 3 g to 30 g sludge dry matter per liter of wastewater.

After the wet oxidation, the acidic wastewater is best neutralized oralkalized by addition of alkali.

Another embodiment of the invention is characterized in that the basicwastewater is passed through a stripping column to remove the ammoniumformed during the wet oxidation of organic nitrogen compounds and torecover it in the form of ammonia solution. The ammonia solutionrecovered may then advantageously be used as reducing agent in thecatalytic removal of nitrogen oxides from waste gases.

Heavy metal hydroxides may be precipitated during the alkalization ofthe acidic wastewater coming from the wet oxidation process. They arebest filtered off in a separate step and separately disposed of. Thebasic wastewater freed from heavy metal hydroxides is thenadvantageously delivered to a biological sewage treatment plant.

A further improvement is obtained by delivering the wastewater free fromthe ammonium to the denitrification stage of a biological sewagetreatment plant.

The following advantages are afforded by the invention:

There is no need for the addition of expensive quinoidal chemicals orfor the expensive production of quinones in a preliminary stage of thewastewater treatment.

At the same time, the problematical treated sludge can be inexpensivelyeliminated.

The ammonia formed during the oxidation can readily be recovered andreused.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a flowchart showing the process of the invention.

The invention is described in more detail in the following with the aidof Comparison Examples and a flow chart.

Treated sludge with a concentration of around 10 to 20 g dry matter perliter wastewater and iron ions in the form of iron salts with aconcentration of around 0.3 to 0.5 g Fe⁺⁺ /1 are added to an untreatedwastewater with a COD content of 1 to 200 g/l and the mixture isacidified with H₂ SO₄ to a pH value of 1.5 to 2. The acidic wastewatersludge mixture is brought by a pump 1 to a process pressure of around 20bar, heated in a countercurrent heat exchanger 2 and then introducedinto the oxidation reactor 3. The oxidation reactor 3 is a bubble columninto which pure oxygen is introduced in the form of fine bubbles throughinjectors at the base of the column. The oxidation reaction preferablytakes place at temperatures of 120° to 220° C. over a residence time of1 to 3 hours. The organic pollutants in the wastewater are oxidized toCO₂ and water. The oxidation reactor 3 is started up by preheating withsteam. The process then continues auto-thermally.

The treated wastewater flows from the head of the oxidation reactor 3 tothe countercurrent heat exchanger 2 where it is cooled to around 100° C.Gaseous components are then separated from the wastewater in a cycloneseparator 4 and relieved of pressure. A small quantity of waste gascontaining CO in addition to CO₂ is formed and can be aftertreated inknown manner.

During the wet oxidation process in the reactor 3, organic nitrogencompounds are converted into ammonium. If the wastewater issuing fromthe separator 4 is alkalized by addition of alkali, for example NaOH,the ammonium can be removed from the wastewater in the form of ammoniain a following stripping column 5. The ammonia recovered mayadvantageously be used as reducing agent in the catalytic removal ofnitrogen oxides from waste gases.

During the wet oxidation process in the reactor 3, any heavy metalspresent in the wastewater also pass into solution. They are convertedinto hydroxides during the alkalization step and, after cooling in theheat exchanger 6, may be separated as solids in a sedimentation vessel7. The solid separated off may then be freed from water in a filterpress 8. The dewatered metal hydroxides are then separately disposed ofwhile the liquid phase is returned to the wastewater pipe 9. The treatedwastewater freed from ammonium and heavy metal compounds still containsorganic fragments which are readily biodegradable. For this reason, thewastewater line 9 is connected to a conventional biological sewagetreatment plant where the wastewater can be directly introduced into adenitrification stage optionally present.

EXAMPLE 1 (PRIOR ART)

A mixed wastewater with a total COD (chemical oxygen demand) value of52.5 g/l and an AOX content of 17 mg/l was oxidized with oxygen in anautoclave for 3 hours at 190° C./18 bar. The COD value of 52.5 appliedto the solids-containing untreated wastewater. After removal of thesolids by filtration, the COD value was 48.4. Both values are entered incolumn 2 of Table 1. In this Example, only iron ions were added in aconcentration of 0.3 g/l. The result was a COD reduction to 32.0 g/l(39%) and a reduction in AOX (adsorbed organic halogen compounds) to 3mg/l (82%). These values are shown in column 3 of Table 1.

EXAMPLE 2 (PRIOR ART)

Using the same untreated wastewater with a COD value of 52.5 g/l,quinones were added as co-catalysts in addition to the iron ions and thewet oxidation was then carried out under the same conditions as inExample 1. The quinones were obtained as in DE 33 16 265 by directintroduction of 5 g/l of lignite dust into the heated alkaline untreatedwastewater. The wastewater was then acidified with H₂ SO₄ and oxidizedwith oxygen. The COD value of the wastewater was reduced to 21.0 g/l(60%) by this treatment. The AOX content was reduced to 2 mg/l. Thesevalues are shown in column 3 of Table 1.

EXAMPLE 3 (INVENTION)

In a first test, treated sludge from the digestion tower of a communalsewage treatment plant with a concentration of 6 g dry matter per literwastewater (DM/l) and, in a second test, treated sludge from the samesource with a concentration of 12 g DM/l wastewater was added to theiron-containing mixed wastewater of Example 1. The sludge dry matter hada percentage organic component of around 50%. The wet oxidation wasagain carried out under the same conditions as described in Example 1.

As the results set out in columns 5 and 6 of the Table show, the CODvalue was reduced to 23.3 g/l (56%) and to 20.7 g/l (60.5%) by thecatalytic effect of adding treated sludge. The oxidation result obtainedwas thus the same as that obtained by adding quinone catalysts inaccordance with Example 2. The small differences in COD betweenunfiltered and filtered samples in the second test (20.7 g/l and 20.5g/l) show that the organic sludge component is almost completelyoxidized.

    ______________________________________                                        Wet oxidation of a wastewater mixture by the LOPROX process                   (T = 190° C., p = 18 bar, t.sub.v = 3 h, C.sub.Fe.sup.2+  = 0.3        g/l)                                                                                   Un-              Exam-                                                        treated          ple 2 Example 3                                     Analysis waste-  Example 1                                                                              Fe/qui-                                                                             Fe/treated sludge                             parameter                                                                              water   Fe       nones 6 g DM/1                                                                             12 g DM/1                              ______________________________________                                        COD  q O.sub.2 /l!                                                                     52.5/   32.0/26.7                                                                              21.0/ 23.3/21.2                                                                            20.7/20.5                                       48.4             19.8                                                BOD.sub.5  g O.sub.2 /l!                                                               18.8    14.2     10.4  11.7   12.1                                   TOC  g C/1!                                                                            12.2    10.2     7.4   9.4    9.1                                    Cl.sup.- 5.0/l!  1.7      1.6   1.6    1.6                                    AOX  mg/l!                                                                             17      3        2     3      3                                      pH       8.3     1.7      2.1   2.0    2.1                                    ______________________________________                                    

We claim:
 1. A single-oxidation process for the degradation of organic pollutants in waste water wherein said organic pollutants are oxidized by the addition of oxygen or an oxygen containing gas to said waste water at a temperature of 80°-330° C., a pressure of 3 bar to 50 bar and a pH of less than 7 in the presence of iron ions and surplus sludge or digested sludge from an industrial or municipal sewage treatment plant.
 2. A process according to claim 1, wherein the digested sludge or surplus sludge is added to the wastewater in the presence of the iron ions in a quantity of 1 to 150 g sludge dry matter per liter wastewater.
 3. A process as claimed in claim 1, characterized in that, after the wet oxidation, the acidic wastewater is neutralized or alkalized by addition of alkali.
 4. A process according to claim 3, wherein the basic wastewater is passed through a stripping column to remove ammonium, which is formed during the wet oxidation of organic nitrogen compounds, and recover it as an ammonia solution.
 5. A process as claimed in claim 4, characterized in that the ammonia solution recovered is reused as reducing agent in the catalytic removal of nitrogen oxides from waste gases.
 6. A process according to claim 3, wherein heavy metal oxides, which are precipitated as a result of the addition of the alkali after the wet oxidation in the wastewater, are filtered off and separately disposed of.
 7. A process as claimed in claim 3, characterized in that the neutralized or basic wastewater is fed to a biological sewage treatment plant.
 8. A process as claimed in claim 4, characterized in that the wastewater freed from the ammonium is introduced into the denitrification stage of a biological sewage treatment plant. 